1. Field of the Invention
This invention relates to a detection element for determining an oxygen content, and more particularly, to a detection element adapted for determining an oxygen content of the hot exhaust gas discharged from an internal combustion engine such as an automobile and to a detector for determining the oxygen content using the element.
2. Prior Art
It is a general practice of the automotive industry to reduce exhaust gas pollution by mounting an exhaust pipe with a detector for determining an oxygen content, and by regulating and maintaining an air fuel ratio at a value of about one. In the conventional detectors of this type it is typical to use a solid electrolyte tubular body as a detection element. Such elements have a tubular body being closed at one end and formed integrally with an ion-conductive ceramic material such as stabilized zirconium oxide. Such elements have serious problems in that ion-conductive ceramic materials, such as zirconium oxide are very high in price.
In contrast thereto, it is proposed in the Japanese Patent Application No. 81027/1975 to provide a detection element whose production cost is reduced by fixing a plate-like stabilized zirconium oxide to the concentric hole formed at the end of a tubular body. The tubular body is formed of an ion-nonconductive ceramic material such as folsterite which is inexpensive. The detection element described that the Japanese Patent Application has the following disadvantages. Namely, (a) the end portion of the element is quickly heated by the hot exhaust gas discharged and a sharp temperature gradient is produced immediately after the start of an internal combustion engine. Because folsterite is inferior in thermal shock resistance to zirconium oxide, the detection element is liable to crack at the end of the folsterite tubular body; (b) the flat zirconium oxide plate fixed to the concentric hole at the end of the folsterite tubular body is greater in the percentage of a catalyzing layer such as platinum on the inside and outside surfaces of the plate than the folsterite tubular body. This plate quickly expanded, with the result that the end portion of the folsterite tubular body is subjected by such expansion to centrifugal depression which, in turn, causes more cracks to occur. Above all, when, as in the previous application, the outer circumferential surface of the flat zirconium oxide plate is mounted on the inner circumference of the opening end of the folsterite tubular body, the likelihood of cracking is great.